1. Field of the Invention
The present invention relates to signal transmission, and in particular but not exclusively to wireless signal transmission. Such wireless signal transmission is required, for example, in sensor systems for use with two elements that are movable relative to one another, a sensor on one of the two elements transmitting its sensor data to, and preferably being supplied with power from, a receiver on the other element. For example, one embodiment of the present invention is intended for use in a pressure sensor system on a vehicle to measure tyre pressure.
2. Description of the Related Art
The key problem of in-vehicle tyre pressure measurement stems from the fact that the wheels and tyres rotate relative to the vehicle. Sensed information has to be passed from the moving wheel. Wheels and tyres must still be interchangeable by users and garages and any failures must have safe consequences. Furthermore, tyre pressures must be sensed accurately and reliably, and the sensed information must be converted into a suitable form of signal which is transmitted via a suitable link provided at each wheel. The information must be conveyed to the dashboard and converted into a form suitable for display. An overall accuracy of about xc2x12% should desirably be maintained. In addition, the complete system must be implemented within certain constraints of size and weight to operate in the electronically and environmentally inhospitable environment of the vehicle. To be applicable to mass-market vehicles the system must also be cheap.
Tyre pressures vary significantly with ambient conditions. This means that a measurement of absolute pressure alone is insufficiently accurate to verify that the tyre is correctly inflated. Even a measure of pressure relative to atmospheric pressure is insufficient if the air in the tyre is hot from use. It is therefore also desirable to measure the air temperature in the tyre and to make allowances for this to establish that the tyre inflation is correct.
In one commercially available tyre pressure measurement system, a battery, sensors and a radio transmitter are provided within each tyre on the vehicle, and the vehicle carries a central radio receiving station to interpret and display the data. The transmitters in the wheels are activated by vehicle motion, and each has a coded signature so that it can be identified and transmits its data to the central receiving station where it is interpreted for display. The system can relay both pressure and temperature information. This system, however, has a large number of drawbacks. It is complex and expensive; it requires maintenance of the batteries in the wheels; it uses radio for transmission which is pervasive and has electro-magnetic coupling (EMC) pollution problems at high vehicle density; the system must be reconfigured and recoded if the wheel is moved to a different position; and it does not operate until the wheel is turning and therefore does not operate on spare wheels or stationary vehicles.
Another existing measurement system uses concentric close-coupled transformers on the vehicle axles, and transmits power to the sensors and circuitry in the wheels and multiplexes (times slices) this with information transmitted from the wheels. Each transformer is connected by cable to a central module which controls the system and decodes the information for display. The system is designed primarily for heavy commercial vehicles and will relay both pressure and temperature data.
This system is also undesirably complex and requires that the coupling transformers are incorporated at the vehicle design stage as they must be concentric with the axles; it is complex electronically because of the time slicing; additional connections must be made to a wheel when it is fitted and, while it is acceptable on heavy commercial vehicles, it is problematic on cars.
Another system employs a simple go/no-go sensor in each wheel which changes its characteristic resonant frequency to indicate the change of state. Each sensor is activated by an electromagnetic pulse and its echo is monitored. This system, although simple, offers limited performance. The go/no-go threshold is intrinsic to the sensor and therefore the sensor has to be changed if a different threshold is required, for example if a wheel is to be moved from one axle to another or if a high load is to be carried. The system cannot detect over-pressure, nor is it readily adaptable to multi-wheel axles.
A number of other systems exist which also incorporate tyre re-inflation mechanisms. These are inevitably costly and complicated. Some systems are available which measure other parameters, such as axial height or rolling tyre circumference, to give an indication of the tyre inflation. These other parameters do not easily relate to the tyre manufacturer""s specifications.
Various attempts have been made to provide signal transmission apparatus, adapted for use in tyre pressure sensing applications, in which a wheel-side resonator is coupled inductively to a vehicle-side circuit that applies an excitation signal to the wheel-side resonator. The wheel-side resonator has at least one component whose effective value influences a natural resonant frequency of the resonator and is changed in use of the apparatus as the tyre pressure changes.
These various attempts have in common the feature that the wheel-side resonator and vehicle-side circuit together form a variable-frequency oscillator circuit whose frequency of oscillation (the excitation frequency) varies with variations in the effective value of the variable resonator component. The excitation frequency is then simply equal to the instantaneous resonant frequency of the entire oscillator circuit. The excitation frequency is then generally measured using a frequency meter or the like to produce a tyre pressure indication. WO-A-87/03544 and GB-A-2065896 disclose examples of this kind of apparatus.
In another example, disclosed in DE-A-3203880, a variable-frequency oscillator circuit is formed involving the wheel-side resonator, but in this case the wheel-side resonator serves to damp oscillations in the vehicle-side circuit. When the tyre pressure is at a desired normal value the vehicle-side circuit is maximally damped by the wheel-side resonator and no oscillations take place in the vehicle-side circuit. As the tyre pressure falls below the normal value the effective value of the resonator variable component changes and the excitation frequency changes accordingly. This change in the excitation frequency is accompanied by a reduction in the damping effect so that oscillations take place in the vehicle-side circuit. The amplitude of these oscillations is measured to provide an indication of tyre pressure. These examples suffer from the disadvantage that the excitation frequency may be influenced significantly by other factors unrelated to the change in effective value, so that in many practical applications, especially involving the inhospitable environment of a vehicle, excitation-frequency changes (or damping changes) may not be sufficiently reliable to use for signal transmission purposes.
There are some examples of apparatus in which the excitation frequency is not made to vary with changes in the effective value of the resonator variable component but in these examples the excitation signal does not serve to bring about in the wheel-side resonator oscillations that have the predetermined excitation frequency of the excitation signal itself. These examples rely on applying excitation pulses at predetermined intervals to the wheel-side resonator so as to excite the resonator into oscillation at its instantaneous resonant frequency (which is dependent on the effective value of the resonator variable component). The frequency of the resonator oscillations (an echo signal) is then measured in the quiet periods between excitation pulses. Examples of such echo-based apparatus are disclosed in EP-A-0636502 and WO-A-87/03544. The echo-based examples require a complex time-multiplexing design to generate the excitation pulses in a transmission phase and to detect the echo signal in a separate reception phase, and the echo signal tends to be undesirably weak as the wheel-side resonator can only be excited intermittently.
According to a first aspect of the present invention there is provided signal transmission apparatus comprising: transmitter circuitry including resonator means having at least one component whose effective value influences a natural resonant frequency of the resonator means and can be changed in use of the circuitry; excitation means for applying to the resonator means an excitation signal having an excitation frequency, said excitation signal serving to bring about oscillations having said excitation frequency in said resonator means; and coupling means for providing a coupling between the said resonator means and receiver circuitry of the apparatus, the receiver circuitry being operable to detect such a change in the said effective value via the said coupling;
characterised in that said excitation frequency is a predetermined excitation frequency that does not change when said effective value changes and that differs from each value of said natural resonant frequency of said resonator means as said effective value changes in use of the transmitter circuitry.
According to a second aspect of the present invention there is provided sensing apparatus, for transmitting sensor data from a first element to a second element, the first and second elements being movable relative to one another, which apparatus includes signal transmission apparatus embodying the aforesaid first aspect of the present invention; the said transmitting circuitry being adapted to be carried by the first element and comprising sensor means for sensing one or more predetermined parameters, the said change in the said effective value being brought about by a change in at least one of the said predetermined parameters; and the said receiving circuitry being adapted to be carried by the second element.
According to a third aspect of the present invention there is provided tyre pressure measuring apparatus, adapted to be carried by a vehicle, including signal transmission apparatus, embodying the aforesaid first aspect of the present invention, wherein the said transmitter circuitry comprises sensor means for sensing one or more predetermined parameters, and the said change in the said effective value is brought about by a change in at least one of the said predetermined parameters.
According to a fourth aspect of the present invention there is provided a signal transmission method, for use with transmitter circuitry that includes resonator means having at least one component whose effective value influences a natural resonant frequency of the resonator means, and with receiver circuitry that has a coupling when in use to the said resonator means, the method comprising: applying to the resonator means an excitation signal having an excitation frequency so that oscillations having said excitation frequency are brought about in said resonator means; bringing about a change in the said effective value of the said one component in the transmitter circuitry; and detecting such a change in the said effective value in the receiver circuitry via the said coupling; characterised in that said excitation frequency is a predetermined excitation frequency that does not change when said effective value changes and that differs from each value of said natural resonant frequency of said resonator means as said effective value changes in use of the transmitter circuitry.